I. Technical Field
The present invention relates to a polymer actuator which can achieve large expansion and contraction operations in a stable manner with a simple and inexpensive structure.
II. Description of the Related Art
Along with increasing demands for machines, such as house-service robots, which are operated closely to human beings, there have been ever-increasing expectations for artificial muscle actuators, which can be operated flexibly like human muscles. Candidates for the artificial muscle actuators include an actuator using a conductive polymer, an actuator using a polymer containing carbon fine particles (for example, see JP-A No. 2005-176412), an actuator using a polymer structural member containing carbon nanotubes (for example, see JP-A No. 2005-176428), and the like, and since these actuators utilize a phenomenon in which a structural member containing a polymer material is expanded and contracted in response to movements of ions, they are generally referred to as ionic polymer actuators.
An actuator, as shown in FIGS. 7A, 7B, and 7C, which generates a flexible deformation, has been proposed as one example of the artificial muscle actuator using a conductive polymer, which is a type of ionic polymer actuators. This actuator has a structure in which a solid-state electrolyte molded member 32 serving as an electrolyte retention layer is sandwiched by polyaniline film members 35a and 35b, which are polymer structural members using a conductive polymer. By turning a switch 37 on, a potential difference, set at a power supply 36, is applied between the polyaniline film members 35a and 35b so that, as shown in FIG. 7B, negative ions are inserted into one polyaniline film member 35b so as to be expanded, while negative ions are released from the other polyaniline film member 35a so as to be contracted, with the result that a flexible deformation is generated (for example, see JP-A No. 11-169393).
In this structure, the flexible deformation is generated by a difference of displacement amounts of the polyaniline film members 35a and 35b which are two conductive polymer films serving as electrodes, but in contrast, another structure has been known in which, by forming the solid-state electrolyte molded member 32 serving as the electrolyte retention layer by the use of a liquid or a gel-state material, the deformations of the polyaniline film members 35a and 35b, serving as both electrodes, are made so as not to be influenced by each other so that only the displacement of either one of the conductive polymer films 35a and 35b is taken out, and an actuator which executes expansion and contraction deformations is thus achieved. In this case, no conductive polymer structure is required for the electrode whose displacement is not utilized, and a metal electrode is mainly utilized therefor; however, it is also indicated that the displacement is increased by forming the conductive polymer structure on the metal electrode (for example, see Proceedings of SPIE, Vol. 4695, pages 8 to 16).
The principle by which the ionic polymer actuator is expanded and contracted is derived from not only a volume change due to such an insertion of ions, but also a structural change in polymer, electrostatic repulsion, and the like; and in any of these cases, a structure in which a potential difference is applied between two electrodes connected through an electrolyte retention layer interposed therebetween is used, and mutually corresponding phenomena are generated on the respective electrodes. Since such ionic polymer actuators generate a strain corresponding to that of a muscle upon application of a low voltage in a range of 2 to 3V, they are expected to be put into practical use as an artificial muscle.
However, since the ionic polymer actuator utilizes expanding and contracting operations of a flexible polymer structural member, shapes of the expanded state and contracted state are respectively changed upon application of a load to the actuator, with the result that the expansion and contraction range as the actuator is changed. For this reason, in general positional control, in either of the cases of no load, and application of a load, the operation range needs to be limited to a range capable of being reached by expanding and contracting operations; consequently, it becomes impossible to expand and contract the actuator to the maximum degree.
In order to solve these issues, it is necessary to change an operating state in response to an expanded or contracted state of the actuator. With respect to this method, a method for measuring a charge, a method using a plurality of sensors, a method for continuously applying a constant voltage, and the like are proposed.
The above method for measuring a charge is a method in which, since the displacement in the ionic polymer actuator depends on a charge in a polymer structural member or its corresponding number of ions, the number of incoming and outgoing charges to and from the polymer structural member are measured, thereby evaluating the expanded or contracted state. In this method, however, measuring errors are accumulated because addition and subtraction of the number of the charges are executed each time the actuator is operated. Consequently, a measuring system such as a high precision charge-measuring device needs to be used, resulting in defects of complex circuits and high costs.
Next, the above method using a plurality of sensors includes, for example, a method in which a power sensor is used in addition to a displacement sensor so that the expanded or contracted state is evaluated by using a relationship between a load and an expansion and contraction range which have been preliminarily measured. However, this method requires a power sensor additionally to cause a defect of high costs.
Then, the above method for continuously applying a constant voltage is a method by which a voltage, which is adjusted to such a degree as not to cause degradation of the polymer structural member or the electrolyte retention layer, but to allow the polymer structural member to expand or contract, is continuously applied. With this arrangement, the polymer structural member approaches a predetermined expanded or contracted state as time elapses. In this method, however, it is difficult to determine the timing in which an applied voltage is changed, and there is a defect that, when enough time is given until the displacement in the polymer structural member has positively stabilized, the operation of the actuator becomes extremely slow, while, when the applied voltage is changed earlier, the expanded or contracted state of the polymer structural member is brought into a transient response state, causing degradation of the correlation to the applied voltage. Moreover, in the case when response speeds are different between the expansion side and the contraction side, an issue arises in which, upon changing a voltage periodically to provide reciprocating operations, a drifting phenomenon of displacement toward one of the sides tends to occur.